Fish

Ocean saltwater covers more than three-quarters of Earth's surface;
lakes, rivers, streams, ponds, canals, swamps, marshes, and other forms of
freshwater cover vast expanses of the planet's surface as well. One
of the most successful groups of animals that have evolved to fill all
these habitats are fish.

There are two types of fish: a small group with skeletons made of
cartilage (a bonelike elastic tissue less rigid than true bone) and an
enormous group with skeletons made of bone (like that found in humans).
Cartilaginous fish include the sharks, skates, rays, and dogfish. The
remainder—more than 25,000 species (more than all other species of
vertebrates combined)—are known as bony fish.

All fish are cold-blooded, meaning they do not have a constant body
temperature but take on the temperature of the surrounding water. The
majority of fish species have bodies that are streamlined; their bodies
are covered with tiny, smooth scales that offer no resistance to a
fish's movement through water. The scales themselves are covered
with a slimy coating that further reduces friction. Additionally, a
fish's external appendages (fins) have been reduced to produce
minimal resistance to the water as they propel the fish through it. Fins
fall into two categories: vertical fins, which occur individually, and
paired fins. Examples of the vertical fins are a dorsal fin that runs down
the middle of a fish's back and the anal fin that runs along its
underside. Examples of paired fins are those that appear on either side of
a fish's upper body, below and behind its eyes.

A diver swimming with a stingray in the Grand Cayman Islands.
(Reproduced by permission of

The Stock Market

.)

The form, size, and number of fins varies considerably according to an
individual species' habitat and requirements. In fast-swimming
species, such as the tuna or mackerel, the dorsal and anal fins have thin,
sharp shapes that reduce friction. In puffer or porcupine fish, by
contrast, the fins are greatly reduced—for use in short paddling
movements. Other species, such as eels, have lost almost all traces of
external fins and swim instead by rhythmic movements of their muscular
bodies.

Another important adaptation made by fish is their swim bladder. The swim
bladder is a chamber filled with air that allows a fish to remain at the
same level in water while expending very little energy.

Fish breathe through structures known as gills. When a fish takes in water
through its mouth, the flaps that cover its gills are closed. When the
fish closes its mouth, the flaps open and water is expelled through the
gills. In this process, oxygen dissolved in the water is absorbed into the
fish's bloodstream.

Bony fish are either carnivorous (meat-eating), herbivorous (planteating),
or both. And fish are, of course, one of the world's most popular
foods. In island nations and countries with long coastlines, fish are a
major part of the diet. They are also a healthful food since they are high
in protein and low in fat content.

Cartilaginous fish

The cartilaginous fish—whose skeletons are made of
cartilage—include both sharks and rays. An intriguing
characteristic of sharks is the presence of tiny primitive teeth on their
skin. These denticles are similar in some ways to human teeth, although
much smaller in size. Thus the texture of a shark's skin is similar
to that of fine sandpaper. Human swimmers can be badly cut by coming into
contact with the skin of a shark. The skin of a ray, on the other hand, is
entirely smooth except for the back or upper tail surface, where denticles
have developed into large, strong spines.

The jaw teeth of both sharks and rays are, in fact, modified denticles.
These teeth are lost when they become worn and are replaced by rows of new
teeth from the space behind them. In some species of sharks, the jaw looks
like an assembly line, with new teeth filling spaces immediately.

Like bony fish, both sharks and rays breathe through gills. They also have
an opening called a spiracle on both sides of the head behind the eye. The
spiracle allows water to flow through the gills without taking in large
amounts of mud and sand. This adaptation is especially useful for rays,
which often bury in the sand, and for sharks, which often rest on
the ocean bottom. Unlike the bony fish, sharks and rays do not possess a
swim bladder.

Cartilaginous fish are predatory: they feed on other animals, from
zooplankton to shellfish to whales. And they themselves are sought after
by humans as a food source. Shark meat, once marketed under the pseudonyms
of flake and steakfish, is now popular worldwide. Shark fins have long
been popular in Asia. Rays are considered delicacies in Great Britain and
France, and thornback rays and flapper skates are often sold as sea trout.